The attachment of bacteria to solid surfaces is the first step in the formation of biofilms – communities of sessile microbes surrounded by a polymeric matrix. A growing appreciation of the ubiquity and importance of biofilms in medicine and industry has led to a proliferation of antiadhesion strategies that include chemical, biological, and physical approaches. Attempts to block biofilm formation must also take into account the rapid evolution of bacteria and their ability to resist many chemical assaults. By preventing adhesion in a manner that is nontoxic for the bacteria as well as for the application (e.g. medical implants), there is hope that we may reduce the costs associated with biofilm formation without imposing selective pressures for the development of antibiotic resistance. Physical strategies, particularly the use of rationally designed surface topographies, have gained attention recently as highly nonspecific methods for prevention of attachment without the use of antimicrobials.

Understanding how bacteria interact with surfaces that have roughness on the micrometer and submicrometer length scales comparable with the length scale of the bacteria themselves is critical to the development of antiadhesive materials. Such surfaces are also relevant for a deeper understanding of the native bacterial lifestyle, because most surfaces in nature are not atomically smooth. Indeed, the microvilli of the intestines are between 80 and 150 nm in diameter, creating considerable topographic constraints for enteric pathogens that might attempt to colonize the epithelium. Geometric considerations suggest that surfaces with roughness on the bacterial length scale provide less available surface area and fewer attachment points for rigid bacterial bodies than smooth surfaces. However, the simplistic view of bacteria as rigid rods or spheres ignores the presence of bacterial appendages, such as pili and flagella.

Previous work has demonstrated that type I pili and flagella are required for biofilm formation by Escherichia coli. Flagella have been suggested to aid in overcoming surface repulsive forces and, possibly, to aid in spreading of cells along a surface. Additionally, exopolysaccharides and surface antigens help to develop biofilm morphology. It is possible that such extracellular features also play important roles in allowing bacteria to adapt and adhere to bumpy surfaces. Using wild-type cells and deletions of several biofilm-associated genes, a recent paper in PNAS examines the role of surface appendages on adhesion to patterned substrates and show that flagella in particular appear to aid in attachment to surfaces inaccessible to the cell body, by reaching into crevices and masking surface topography. The results suggest that flagella may provide an structural function in biofilm formation additional to their role in motility.

Bacterial flagella explore microscale hummocks and hollows to increase adhesion. (2013) PNAS USA 110(14): 5624-5629. doi: 10.1073/pnas.1219662110
Biofilms, surface-bound communities of microbes, are economically and medically important due to their pathogenic and obstructive properties. Among the numerous strategies to prevent bacterial adhesion and subsequent biofilm formation, surface topography was recently proposed as a highly nonspecific method that does not rely on small-molecule antibacterial compounds, which promote resistance. Here, we provide a detailed investigation of how the introduction of submicrometer crevices to a surface affects attachment of Escherichia coli. These crevices reduce substrate surface area available to the cell body but increase overall surface area. We have found that, during the first 2 h, adhesion to topographic surfaces is significantly reduced compared with flat controls, but this behavior abruptly reverses to significantly increased adhesion at longer exposures. We show that this reversal coincides with bacterially induced wetting transitions and that flagellar filaments aid in adhesion to these wetted topographic surfaces. We demonstrate that flagella are able to reach into crevices, access additional surface area, and produce a dense, fibrous network. Mutants lacking flagella show comparatively reduced adhesion. By varying substrate crevice sizes, we determine the conditions under which having flagella is most advantageous for adhesion. These findings strongly indicate that, in addition to their role in swimming motility, flagella are involved in attachment and can furthermore act as structural elements, enabling bacteria to overcome unfavorable surface topographies. This work contributes insights for the future design of antifouling surfaces and for improved understanding of bacterial behavior in native, structured environments.